Add a pull request template

.github/pull_request_template.md

@@ -0,0 +1,31 @@
+# Before your submission:
+
+* Ensure your PR follows the [contribution
+  guidelines](https://github.com/leanprover/LNSym/blob/main/CONTRIBUTING.md).
+
+* Remember to document and comment your code.
+
+* Remove this section, up to and including the `---` before submitting.
+
+---
+
+### Issues:
+
+Resolves #<issue_number>.
+
+Remove this section if this PR doesn't address any open issues.
+
+### Description:
+
+A detailed description of your contribution. Why is this change
+necessary?
+
+### Testing:
+
+What tests have been run? Did `make all` succeed for your changes? Was
+conformance testing successful on an Aarch64 machine?
+
+### License:
+
+By submitting this pull request, I confirm that my contribution is
+made under the terms of the Apache 2.0 license.